1. Field of the Invention
The present invention relates to a liquid crystal display device of an active matrix type provided with a matrix of picture element electrodes and switching devices and, more specifically, to a liquid crystal display device of an active matrix type incorporating a structure capable of suppressing the inverse orientation of the liquid crystal molecules or reverse tilt that occurs along the edges of the picture element electrodes.
2. Description of the Related Art
The construction of a general liquid crystal display device of an active matrix type will be described with reference to FIG. 13 to facilitate understanding the present invention.
A transmissive type liquid crystal display device comprises two polarizers 110 and 111, and a liquid crystal panel sandwiched between a pair of the polarizer 110 and 111, and comprises a first glass substrate 101, a second glass substrate 102 disposed opposite to the first glass substrate 101, and a liquid crystal 103 sealed in the space between the two glass substrates 101 and 102. Formed on the inner surface of the first glass substrate 101 are signal lines 104, address lines 105 extending in a direction perpendicular to the signal lines 104, thin film transistors (TFTs) 106, i.e., switching devices, which are arranged in a matrix in areas demarcated by the signal lines 104 and the address lines 105, and picture element electrodes 107, which are arranged in a matrix in the areas demarcated by the signal lines 104 and the address lines 105. Formed on the inner surface of the second glass substrate 102 are a common electrode 108, and R, G and B color filters 109.
When driving the liquid crystal display panel, selection signals having a pulse width corresponding to a horizontal scanning period are applied sequentially to the address lines 105. In a period where one of the address lines 105 is selected, sampled image signals are held on the signal lines 104, and then, image signals are written in the corresponding picture element electrodes 107. The image signal written in each picture element electrode 107 is held for a period corresponding to one field, and then a signal of the opposite polarity is written in the picture element electrode 107 for the next field. Thus, the liquid crystal 103 is driven by alternate signals.
Electric fields created between the signal lines 104 and the picture element electrodes 107 and between the address lines 105 and the picture element electrodes 107 disturb the molecular orientation of the liquid crystal. Such irregular molecular orientation of the liquid crystal is called reverse tilt. Referring to FIG. 14, the signal line 104 or the address line 105, and the picture element electrode 107 are formed on the first glass substrate 101, and the surface of the first glass substrate 101 is coated with a alignment layer 112. A lateral electric field substantially parallel to the surface of the first glass substrate 101 is created between the signal line 104 (the address line 105) and the picture element electrode 107 and the orientation of the molecules 113 of the liquid crystal is changed forcibly by the electric field. Consequently, strain is induced therein and elastic energy is concentrated. Since it is possible that the interaction of the molecules 113 of the liquid crystal induces strain energy in the picture element electrode 107, an abnormal domain 114 of an orientation different from that of most part of the picture element electrode 107 is formed in the picture element electrode 107 as shown in FIG. 15. Such a phenomenon is called reverse tilt.
As shown in FIG. 15, the abnormal domain 114 is formed by reverse tilt along the edges of the picture element electrode 107 extending along the signal line 104 (or the address line 105). A disclination occurs along the boundary between the abnormal domain 114 and the normal domain 115 and an emission line appears along the disclination line. If a picture element is displayed in black against the white background, the contrast ratio of the device is reduced because the emission line increases the transmissivity of the black picture element. Various reverse tilt suppressing means have been proposed, none of which, however, has practically been effective.
For example, a reverse tilt suppressing means disclosed in Japanese Patent Laid-open (Kokai) No. Hei 1-266512 removes beforehand a region of a picture element electrode in which reverse tilt is liable to occur or covers a region of a picture element electrode in which reverse tilt is liable to occur with a black mask. Such a reverse tilt suppressing means, however, reduces the aperture ratio of the device because the effective area of the picture element electrode is reduced.